1. Field of Invention
The present invention relates to an amplifier, and more particularly to a resonant amplifier.
2. Related Art
The phenomenon of high-frequency gain roll-off is often encountered in the design of wide-band amplifiers. For the application of the 50 to 860 MHz low noise amplifier in TV tuner front-end, this problem can be overcome by using external passive elements to provide much higher high-frequency impedance so that the gain is boosted at high frequency. However, in recent years, it has become a trend in the field to provide totally integrated-circuit (IC) solutions to solve the problem of high-frequency gain falling.
Conventionally, an amplifier adopts common source architecture with a circuit as shown in FIG. 1. A resistor R1 is mainly used to generate a direct current (DC) bias. The input voltage Vi is input from the gate of a transistor M1, and then is amplified. The amplified voltage Vo is output from a drain of the transistor M1. As the transistor M1 is a metal-oxide semiconductor field-effect transistor (MOSFET), in practice, the input and output stray capacitance may limit the operating frequency range of the amplifier.
When the gain is still inadequate, the same single-stage amplifying circuit architecture can be connected in series to form a multi-stage amplifying circuit. As such, the operating frequency range of the amplifier is more limited.
Furthermore, another amplifier adopts cascode architecture, as shown in FIG. 2. The cascode amplifying circuit comprises a common source transistor M2 together with a common gate transistor M3. The major characteristic is reducing the Miller effect and improving frequency response performance. Though the signal passes through the common source and the common gate transistors, in fact, only the common source transistor provides voltage gain.
Another conventional amplifier for gain boosting employs common gate (base) architecture, as shown in FIG. 3, where the input voltage Vi is input from a gate of a transistor M4, and then is amplified. The inherent stray capacitance between the gate and the source of a transistor M5 acts as the capacitive reactance of the resonant cavity, and a resonant inductor Lr, a blocking load inductor Ld, a bulk bypass capacitor C2, and a bulk coupling capacitor C1 are added to achieve resonance. A bias resistor R2 having blocking effect is also required at the gate of the transistor M5 to form the resonant cavity. However, the reverse isolation between the output and the input will be deteriorated due to R2, so it is better to cascade a common gate (base) transistor M6 with its gate (base) grounding, as shown in FIG. 4. This architecture is disadvantageous for low-voltage operation.
In view of the above, the conventional amplifier still has room for improvement, in order to further provide simple and practical circuit architecture for solving the problem of limited gain in the conventional architecture under the original power consumption.